Methods for thermally determining mass flow of a gas and corresponding thermal mass flow meters are known per se. They operate by heating a first temperature sensor flowed around by the medium, in order to produce, respectively to maintain, a temperature difference relative to a second temperature sensor flowed around by the medium.
The greater the mass flow, the more heat is transported away by the medium, and the more heating power is required, in order, for example, to maintain a predetermined temperature difference. As a result, the so called power coefficient, thus the ratio between the heating power Q and the achieved temperature difference, is a measure for the mass flow.
For exact determination of the mass flow, the power coefficient is used to ascertain a heat transfer coefficient (htc), based on which then via the Nusselt-, Prandtl- and Reynolds number, the mass flow is determined. Details concerning this are disclosed especially in European patent EP 0 624 242 B1. Offenlegungsschrift (laid open Germanapplication) DE 10 2005 057 688 A1 discloses a method for determining mass flow using a Mach number correction.
Both aforementioned documents of the state of the art describe good methods for sufficiently low flow velocities, respectively for symmetric arrangements of temperature sensors.
Especially when asymmetries occur between the temperature sensors at higher flow velocities, the so called recovery effect must be taken into consideration. The recovery effect brings about a warming of the temperature sensor due to stopping of the gas stream on the probe surface. Especially in the case of asymmetric flowmeters, the recovery effect negatively influences the accuracy of measurement considerably. It is, consequently, an object of the present invention to provide a method for thermally determining the mass flow of a gas and to provide a thermal mass flow meter, whereby the recovery effect is corrected.